1. Field of the Invention
The present invention relates to a coupling system for coupling a plurality of laser diodes to a single optical fiber, which provides increased pump light brightness to optical fiber amplifier in order to provide fiber amplifiers with increased power.
2. Description of the Prior Art
Various fiber optical amplifiers are known. Such fiber optic amplifiers normally include an optical fiber and a source of pump light, which excites the active material in the fiber core to a relatively high-energy metastable state. Various techniques are known for coupling the pump light, for example, from a laser diode, to an optical fiber. These techniques are disclosed in U.S. Pat. Nos. 5,127,066; 5,266,978; 5,436,990; 5,636,069; 5,734,766; and 5,745,519, hereby incorporated by reference. These techniques attempt to compensate for the different numerical apertures associated with different planes of the laser diode. An exemplary diode laser bar stack assembly is illustrated in FIG. 1 and generally identified with the reference number 20. As shown, the assembly 20 includes a plurality of laser diodes, known as stripes, generally indicated by the reference numeral 22. In an XYZ coordinate system, the stripes 22 emit light along an axis parallel to the X axis, perpendicular to the plane of the page as shown in FIG. 1, defining a plurality of optical axes. In the XY plane, the beam divergence (i.e., angle between opposite beam edges) is relatively small relative to the optical axis, for example, xc2x15 degrees defining a slow plane. However, in the XZ plane, the beam divergence is substantially larger, for example, xc2x120 degrees relative to the optical axis, defining a fast plane. The problem with coupling such laser diodes to an optical fiber relates to the difference in the beam divergence or numerical aperture in the orthogonal planes of the stripes 22.
The light emitting apertures of such laser diodes or stripes are typically in the range of 1 micron high by 50 to 200 microns wide. The diameter of typical optical fibers is also in the order of microns. As such, coupling the light from the laser diode having a relatively large numerical aperture in the fast plane to an optical fiber having a diameter in the micron range is relatively complicated and requires relatively precise alignment.
Various attempts have been made to optimize the optical coupling between the laser diodes and an optical fiber. Examples of these attempts are disclosed in U.S. Pat. Nos. 5,127,068; 5,268,978; 5,436,990; 5,579,422;5,629,997; 5,636,069; 5,734,766; 5,745,519; and 5,832,150. In particular, U.S. Pat. Nos. 5,127,068; 5,436,990; and 5,579,422 disclose the use of a small diameter multimode optical fiber with a relatively small numerical aperture, used as a microlens, to collimate the output from the laser diode. The microlens is positioned along a longitudinal axis of the diode laser bar assembly, generally perpendicular to the optical axes. The diameter of the microlens is selected to be 20 to 50 percent larger than the aperture of the laser diodes in the slow plane. A plurality of rectangular cross section optical fibers are butt coupled to the microlens. The numerical aperture of the optical fibers is selected to have roughly the same numerical aperture of the laser diode in the slow plane. One of the rectangular cross section optical fibers is aligned with each laser diode in the laser diode bar assembly.
U.S. Pat. Nos. 5,268,978; 5,629,997; 5,636,069; 5,734,766; and 5,832,150 also disclose optical couplers for coupling the light emitted from one laser diode into an optical fiber. Unfortunately, the power of a fiber optic amplifier is limited by the amount of pump light that is injected into the inner cladding of the dual-clad doped fiber media. As such, fiber amplifiers which utilize diode laser bar assemblies are limited in their output power since known coupling methods are limited to coupling the light from a single laser diode and such diode laser bar assemblies to an optical fiber.
U.S. Pat. No. 5,825,551 illustrates another attempt to increase pumping brightness from a diode laser bar assembly. In this embodiment, an optical system is used to reformat the output of the diode laser bar assembly from a horizontal array of horizontal stripes into a vertical column of horizontal stripes. Unfortunately, such a system requires a relatively complicated optical system. Thus, there is a need to provide an optical coupling arrangement that allows for increased light to be pumped from a diode laser bar assembly to an optical fiber to provide increased power fiber amplifiers.
Briefly, the present invention relates to a coupling system for coupling light emitted from a plurality of laser diodes to a single optical fiber, which provides increased pump light brightness to the optical fiber to provide increased power fiber amplifiers. In particular, the system takes advantage of the brightness mismatch in the fast and slow planes of a laser diode to allow capture of more than one laser diode""s power into a single multimode pump fiber. A first cylindrical lens is disposed to be parallel to a row of a laser diodes in a laser diode bar assembly to collimate light from the fast planes of the diodes or stripes. A second cylindrical lens is disposed orthogonal to the optical axis and perpendicular to the first cylindrical lens and is used to image the light from the stripes in the slow plane. A collection lens is provided to image the light from the slow plane as well as collect light from the fast plane and directs light from multiple stripes into a single multimode optical fiber. By increasing a number of stripes that could be coupled to a single optical fiber, the power of the fiber amplifier can be increased.